Image segmentation is the process of partitioning an image into a set of non-overlapping parts, or segments, that together constitute the entire image. Image segmentation is useful for many applications, one of which is compression. For example, it is well known that if an image made up of pixels with the image represented by pixel color values for each of the pixels, the image representation can be compressed if contexts for the pixels can be extracted and used in representing the pixel color values. Thus, if a pixel color value can be one of 256 color values, it can be represented by eight bits, but if a pixel color value is more likely to be a given color value in view of surrounding color values (e.g., in many images, a pixel surrounded by a field of blue pixels is likely to be a blue pixel), then those pixel color values can be represented, on average, in fewer than eight bits. Compression results are likely to be improved if the pixels used for context are actually the context of the pixel being compressed and not pixels that are independent of the pixel being compressed.
Segmentation is helpful in determining which pixels are likely to be related enough to other pixels such that related pixels are used for context with more weight than unrelated pixels. While segmentation is useful for compression, segmentation is also useful for other applications, some of which are mentioned here. For example, a computer program that is not doing compression might process each segment of an image in a manner specific to that segment or associate different data, such as labels, with different segments. In any case, correctly segmenting the image is important for proper operation of the process being performed on the image.
As the terms are used herein, an image is an array of pixel color values such that pixels placed in an array and colored according to each pixel""s color value would represent the image. Each pixel has a location and can be thought of as being a point at that location or as a shape that fills the area around the pixel such that any point within the image is considered to be xe2x80x9cinxe2x80x9d a pixel""s area or considered to be part of the pixel. The image itself might be a multidimensional pixel array on a display, on a printed page, an array stored in memory, or a data signal being transmitted and representing the image. The multidimensional pixel array can be a two-dimensional array for a two-dimensional image, a three-dimensional array for a three-dimensional image, or some other number of dimensions.
The image can be an image of a physical space or plane or an image of a simulated and/or computer-generated space or plane. In the computer graphic arts, a common image is a two-dimensional view of a computer-generated three-dimensional space (such as a geometric model of objects and light sources in a three-space). An image can be a single image or one of a plurality of images that, when arranged in a suitable time order, form a moving image, herein referred to as a video sequence.
When an image is segmented, the image is represented by a plurality of segments. The degenerate case of a single segment comprising the entire image is within the definition of segment used here, but the typical segmentation divides an image into at least two segments. In many images, the segmentation divides the image into a background segment and one or more foreground segments.
In one segmentation method, an image is segmented such that each segment represents a region of the image where the pixel color values are more or less uniform within the segment, but dramatically change at the edges of the image. In that implementation, the regions are connected, i.e., it is possible to move pixel-by-pixel from any one pixel in the region to any other pixel in the region without going outside the region.
Pixel color values can be selected from any number of pixel color spaces. One color space in common use is known as the YUV color space, wherein a pixel color value is described by the triple (Y, U, V), where the Y component refers to a grayscale intensity or luminance, and U and V refer to two chrominance components. The YUV color space is commonly seen in television applications. Another common color space is referred to as the RGB color space, wherein R, G and B refer to the Red, Green and Blue color components, respectively. The RGB color space is commonly seen in computer graphics representations, along with CYMB (cyan, yellow, magenta, black) often used with computer printers.
An example of image segmentation is illustrated in FIG. 1. There, an image frame 10 contains an oval 12, a rectangle 14 and a triangle 16 on a background 18. The image can be segmented into segments based on colors (the shading of the oval 12 in FIG. 1 represents a color distinct from the colors of the rectangle 14, the triangle 16 or the background 18). Thus, the oval 12, the rectangle 14, the triangle 16 and the background 18 are segmented into separate segments in this example.
In this example, if each segment has a very distinct color and the objects in the image 10 end cleanly at pixel boundaries, segmentation is a simple process. In general, however, generating accurate image segments is a difficult problem and there is much open research on this problem, such as in the field of xe2x80x9ccomputer visionxe2x80x9d research. One reason segmentation is often difficult is that a typical image includes noise introduced from various sources including, but not limited to, the digitization process when the image is captured by physical devices and the image also includes regions that do not have well-defined boundaries.
There are several ways of approaching the task of image segmentation, which can generally be grouped into the following: 1) histogram-based segmentation; 2) traditional edge-based segmentation; 3) region-based segmentation; and 4) hybrid segmentation, in which several of the other approaches are combined. Each of these approaches is described below.
1. Histogram-Based Segmentation
Segmentation based upon a histogram technique relies on the determination of the color distribution in each segment. This technique uses only one color plane of the image, typically an intensity color plane (also referred to as the greyscale portion of the image), for segmentation. To perform the technique a processor creates a histogram of the pixel color values in that plane. A histogram is a graph with a series of xe2x80x9cintervalsxe2x80x9d each representing a range of values arrayed along one axis and the total number of occurrences of the values within each range shown along the other axis. The histogram can be used to determine the number of pixels in each segment, by assuming that the color distribution within each segment will be roughly a Gaussian, or bell-shaped, distribution and the color distribution for the entire image will be a sum of Gaussian distributions. Histogram-based techniques attempt to recover the individual Gaussian curves by varying the size of the intervals, i.e., increasing or decreasing the value range, and looking for high or low points. Once the distributions have been ascertained, the each pixel is assigned to the segment with its corresponding intensity range.
The histogram method is fraught with errors. The fundamental assumption that the color distribution is Gaussian is at best a guess, which may not be accurate for all images. In addition, two separate regions of identical intensity will be considered the same segment. Further, the Gaussian distributions recovered by the histogram are incomplete in that they cut off at the ends, thus eliminating some pixels. Further, this method of segmentation is only semi-automatic, in that the technique requires that the number of segments are previously known and that all of the segments are all roughly the same size.
2. Traditional Edge-Based Segmentation
Traditional edge-based segmentation uses differences in color or greyscale intensities to determine edge pixels that delineate various regions within an image. This approach typically assumes that when edge pixels are identified, the edge pixels will completely enclose distinct regions within the image, thereby indicating the segments. However, traditional edge detection techniques often fail to identify all the pixels that are in fact edge pixels, due to noise in images or other artifacts. If some edge pixels are missed, some plurality of distinct regions might be misidentified as being a single segment.
3. Region-Based Segmentation
Region based segmentation attempts to detect homogenous regions and designate them as segments. One class of region-based approaches starts with small uniform regions within the image and tries to merge neighboring regions that are of very close color value in order to form larger regions. Conversely, another class of region-based approaches starts with the entire image and attempts to split the image into multiple homogeneous regions. Both of these approaches result in the image being split at regions where some homogeneity requirements are not met.
The first class of region based segmentation approaches is limited in that the segment edges are approximated depending on the method of dividing the original image. A problem with the second class of region based approaches is that the segments created tend to be distorted relative to the actual underlying segments.
4. Hybrid Segmentation
The goal of hybrid techniques is to combine processes from multiple previous segmentation processes to improve image segmentation. Most hybrid techniques are a combination of edge segmentation and region-based segmentation, with the image being segmented using one of the processes and being continued with the other process. The hybrid techniques attempt to generate better segmentation than a single process alone. However, hybrid methods have proven to require significant user guidance and prior knowledge of the image to be segmented, thus making then unsuitable for applications requiring fully automated segmentation.
The present invention provides an exemplary method and apparatus for digital image segmentation. For example, in one embodiment, a plurality of segments are created from a digital image frame using grid-based segmentation.
Accordingly, in one embodiment, a digital image comprised of an array of pixels each having a pixel location and a pixel color value, is divided into regularly ordered segments. A method comprises obtaining an image frame, assigning an initial segment identifier to each pixel in the image frame independent of each pixel""s color value, testing, using an appropriateness test, pixels for possible reassignment from a current segment to a neighboring segment, and if the appropriateness test indicates a pixel should be reassigned, reassigning the segment identifier of the pixel.
In another embodiment, a digital image is divided into segments, wherein the digital image comprises an array of pixels each having a pixel location and a pixel color value. A method comprises obtaining an image frame comprising an array of pixels each having a pixel color value, overlaying a logical grid over the image data such that initially each subsection of the grid encompasses at least two pixels, and adjusting the grid subsection boundaries to create at least one segment.
A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.